Video and audio productions often comprise multiple independent tracks. For example, a typical motion picture comprises a single video track, and two or more audio tracks. During the creation process for such productions, technicians frequently deal with more tracks than will be used in the final production. In the editing phase of the creation, editors separately edit these individual tracks to produce desired outcomes, but they remain mindful of the combined effect.
In the course of editing video and audio productions it is often important that one track stays synchronised with another. For example, the video of an actors lip movements is commonly closely synchronised with the relevant audio relating the actor's words. As time related aspects of a “source” video track relating to the actors lips are edited (deleted, inserted or moved etc.) the related (synchronised) “dependent” audio is correspondingly edited in cases where is desired to maintain this synchronised relationship.
In many computer-based non-linear editors the editing systems display a visualisation, often referred to as a timeline, of the multiple tracks being edited. This mechanism enables a user to observe a loss of synchronisation between tracks and to manually restore the relationship by editing the dependant tracks. This is however a tedious and error prone approach, since the user needs to continually adjust one or other track to take account of relative changes between the tracks.
Some systems allow synchronisation between different tracks to be maintained by ‘locking’ together the separate tracks. This enables the system to automatically preserve a synchronised timing relationship between the tracks. For some track types this locked type of synchronisation is, however, not desirable. Thus, for example, tracks such as background music or overlay video while dependent on a source video track, should be re-generated whenever the source track changes, rather than precisely reflecting the changes to the source track.
For example, consider the required changes to a dependent audio background music track, and a dependent audio dialog track when an associated source video track is edited. If some content is deleted between time T1 and time T2 from the source video track, the corresponding content needs to be deleted from the dependent dialog track. This corresponding content should not, however, be deleted between these time instants in the dependent background music track, since to do so would cause a discontinuity in the background music track. A more palatable solution is for the duration of the background music track to be shortened by T2−T1. Similarly, the dependent background music track may have its duration increased in response to content being inserted in the source video track, possibly requiring looping if the total duration of the dependent background music track is not sufficient.
Mechanisms for providing automatic synchronisation in current systems are complex, for one or more of the following reasons:                If synchronisation of a dependent track, with reference to a region of a source track, is possible at all, it is generally necessary to define both a beginning and end position of that region of the source track to which the dependent track is to be synchronised.        The aforementioned regions of the source track cannot simply be redefined while preserving properties of the generated dependent track. For example the definition of the media to be used for the generated dependent track is not preserved. In other words, the region of the source track to which the dependent track is synchronised cannot be redefined without removing the synchronisation entirely, and defining it anew. This redefinition requires the attributes of the synchronisation, including which media is to be used in the generation of the relevant portion of the dependent track, to be redefined.        Users typically need to consider how the synchronisation method is operating, and need to incorporate this factor in their editing rather than being able to focus on the key task of editing tracks.        
There are also problems in integrating automatic synchronisation in systems that provide manual post-editing of automatically generated productions. Typically these systems are designed for users with limited experience of computers and/or video editing. Such users may simply desire the convenience of a system that is simple to use. In these systems, the automatic editing rules have been designed by professionals to ensure a good result, but the systems lack robust mechanisms to ensure that the intent of the professional is preserved throughout the editing process.